FORMULA FIERO idle question

I have an 88 formula fiero that has finally not past emissions. It has had a problem with idling when at a stop light. Once u put it into gear it wants to die unless u give it some gas. It sounds like their is a knocking noise coming from the catalitic converter but we've had that changed a couple times. What could be the problem and what could we to if anything as a fast fix for emissions. Car has 120,000 and is in perfect condition still. It drives fine once u get going but once u stop it wants to die. any ideas.

Check engine light ever come on?

check the air filter and the muffler